1. Field of the Invention
This invention relates to blind fasteners; and, particularly to those used in the aircraft industry.
2. Description of the Prior Art
Blind fasteners used in securing sheets of abutting material are well known in the art, particularly in the aircraft industry. Such fasteners generally include a nut, a bolt and a sleeve. The nut has a body with a threaded axial bore therethrough, a chamfered nose at one end and a head at the other. A bolt is threaded in the nut body, the bolt having a head at one end. A hollow cylindrical sleeve surrounds the threaded bolt shaft between the chamfered nose of the nut and the bolt head. In installation, the abutting sheets are of varying thicknesses and the fasteners are mounted in aligned holes in the sheets. As the threaded shaft of the bolt is drawn through the nut body, the sleeve is pushed by the bolt head and rides up and over the chamfered end of the nut and abuts against the sheets thus locking the fastener in position.
Certain of these fasteners have a break groove at a predetermined location along the threaded shaft. The overall thickness of the sheets varies. Due to the preset location of the break groove, various degrees of installation conditions may result, varying from a minimum grip to a maximum grip. In either case, when the sleeve hits or abuts against the abutting sheet of the installation, break off of the stem or threaded shaft occurs at the break groove.
In a minimum grip situation, the break groove, at break off, is located higher along the stem or threaded shaft and closer to the surface of the head of the nut body. Generally speaking, one locates this break groove so that it breaks off about 0.000 to -0.098 inches with respect to the surface of the nut head. Thus, if the installed condition of the fastener is at the maximum grip condition, the shaft will break off below the surface of the nut head and thus the installation will not be as strong as in a minimum grip condition (where the shaft break off is substantially flush with the surface of the nut head). Accordingly, a weak installation occurs.
One solution to this situation is to relocate the break groove or move it up the shaft so that it breaks off either at or above the nut head surface, about 0.000 to +0.098. However, in such an arrangement, it may be necessary to shave off the protruding tip or pintail of the broken off shaft after installation which entails considerable man hours.
There is a need for a fastener which allows for flush break throughout the entire grip range eliminating the need for machining of pintails yet provides a grip of the installation materials in which the fastener is installed.